In gas, air laid processes for making particulate paper, mat, blanket, board, etc., particles such as fibers are carried in an air stream that is contained and guided by at least a partial enclosure to a permeable collecting surface or screen where the particles are laid down on the collecting surface while the entraining air stream is sucked through the permeable collecting screen. Such a process is used to make fibrous insulation products such as glass fiber insulation bats, blankets, boards, etc. as disclosed in U.S. Pat. Nos. 4,832,723; 4,058,386; 3,901,675; 3,759,680; 3,250,602; 3,030,659 and 2,431,205, the disclosures of which are hereby incorporated by reference.
These processes usually involve spraying water and/or a binder onto the particles while they are airborne in route to collection. This causes the particles to become sticky or tacky, particularly when the entraining air is warm or hot, which is frequently the case. Confining walls containing or guiding the sticky air entrained fibers, the forming chamber, cause a problem because the fibers get blown into contact with the walls and many stick and then catch other particles and build up on the walls. Once these build-ups reach a certain size, portions of each build-up, clumps, break loose due to the air stream rushing past, are picked up by the air stream and carried to the collection screen where they are deposited on the mat, often near one of its edges, but also across the mat. These clumps are often wetter than the rest of the blanket having collected water and binder droplets from the air stream due to their residence time on the walls. The clumps also cause thick areas on the blanket or mat. The thicker areas and the wetter clumps prevent total drying and curing of the binder resin, usually phenolic resin, in the oven later in the process resulting in wet spots in the finished product requiring much otherwise good product to be scrapped.
Also, it is necessary to shut the process down frequently, every six weeks or sooner, and clean the walls and bottom of the forming chamber to reduce the frequency of wet spots. Every shut down is costly.
In the past, things have been done to try to prevent clumps from forming on the forming chamber walls. For example, it has been proposed to use a large conveyor belt for each wall such that each wall is slowly moving. In this way build-ups on the walls are continuously carried out of the forming chamber where, in the return path, the belt, or moving wall, can be continuously cleaned without having to shut the process down. While this system is an improvement, the system is very costly to install and operate and the process material losses are still high because all material that sticks to the walls is carried out, removed in the cleaning step and discarded.
Another way of keeping the sidewalls clean is to continuously wash the walls with a sheet of moving water while the process is operating. This eliminates most of the wet spots from occurring, but the water usage and water cleanup or disposal costs are very high. Further, the process material losses are very high because all particles that contact the moving sheets of water are picked up and carried out of the process with the water.
This problem is becoming more costly and difficult to abide as the regulations on the disposal of wastes becomes more stringent. The need for a process that avoids dirty walls in the forming chamber is thus great. Also, there is a need for improved efficiency in the binder application step since all binder that does not end up on the parties/fibers is removed in the air stream and must be removed by costly filtering, scrubbing and water treatment techniques.